1. Field of the Invention
The invention relates generally to Local Interconnect Networks (LINs), and particularly to a method and apparatus to determine a message time budget on a LIN bus.
2. Description of the Related Art
The Local Interconnect Network (LIN) standard has been created by a group of automotive companies as a means to provide low-cost networks in vehicles. The reduced cost of LIN components is, in part, due to a master-slave single bus with no expensive crystals or resonators on slave modules for clock generation or synchronization.
Messages on LIN networks have known lengths that may be determined by examining the header of each message. Slave nodes determine the communication rate on a LIN network by analyzing a sychronization (SYNC) byte at the beginning of each received message. Knowing the message length and the rate of transmission, slave nodes know the amount of time it should take to receive the entire message. However, since slave nodes do not contain expensive crystals or ceramic resonators for accurate timing, the LIN specification provides that a time budget of 140% of the expected transmission time be given to complete reception of a message. If the message is not received within the budgeted time, the message is rejected.
Current methods employed to calculate a LIN message budget rely upon the measurement of a single bit time from the SYNC byte in the header of each message. This method results in inaccurate message budget calculations that may lead to the improper rejection of valid messages. For example, in the case of a LIN network operating at 19.2 KHz transmitting an eight byte message, the single bit time measurement calculation yields an error of 7.5 microseconds. Consequently, any valid message that completes transmission during the error period is improperly rejected. Therefore, a need exists for an improved method of calculating LIN time budgets that reduces the number of improper message rejections.
In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced.